Wisconsin officials found elevated levels of toxic heavy metals near a frack sand mine spill that sent millions of gallons of sludge into a tributary, carrying it downstream into the Mississippi River.

Department of Natural Resources test results showed high levels of arsenic, lead and other contaminants at the source of a spill at the Hi-Crush mine near Whitehall in western Wisconsin. The mine drained an estimated 10 million gallons of water (38 million liters) last month to rescue a contractor who became trapped inside of a bulldozer for more than two hours after sliding into a pond.

The rush of water tinted waterways orange, flooding fields as the plume swept downstream into a wildlife refuge and the Mississippi River a few days later.

Testing conducted within hours of the spill showed lead concentrations more than 10 times the allowable levels for drinking water, and arsenic at levels nearly seven times the safe levels, according to the results released late Wednesday. Testing also found far higher levels of mercury than in results collected upstream. But those levels dropped off significantly 50 yards (46 meters) downstream in the Trempealeau River.

“The lead is concerning because kids are so sensitive to it,” said Crispin Pierce, director of University of Wisconsin-Eau Claire”s environmental public health program. He added that people should not swim or fish near the spill site. “These are dangerous levels.”

The harmful health impacts of lead exposure are well-documented. It can hinder physical and mental developmental in children, causing learning and attention deficits. In adults, lead exposure can cause kidney damage.

DNR spokesman Jim Dick said samples taken from the Trempealeau River downstream from the spill met state surface water standards. The department has observed no fish kills and he said oxygen levels there can support aquatic life.

But Pierce said that the high turbidity and aluminum levels found in the Trempealeau River might diminish water clarity, making it difficult for fish to find food and reduce sunlight for aquatic plants.

The pond spillage at the 1,447-acre mine was mostly water, sand, clay and silt, according to Hi-Crush officials who”ve maintained the spill has had no health or environmental impacts.

Still, it”s raised questions for residents and local officials who been awaiting the tests to find what”s in the water and if it”s dangerous to human health and the environment.

Trempealeau County Board Chairman Tim Zeglin said the county will meet with state health and DNR officials about spill impacts in the coming days. The DNR is continuing to study rivers and waterways in the area and is reviewing any potential long-term effects on aquatic life.

Hi-Crush said in a statement to The Associated Press on Thursday that it”s making steady progress toward cleaning up the area and remediation.

“We do not expect further sample results to show any issues of concern for the community,” the statement said.